1. Field of the Invention
This invention relates to the techniques of curving glass sheets, in which the glass sheets are brought to the curving tools or molds by a conveyor which defines a substantially horizontal transportation plane for the glass, on which the glass sheets travel one after another, heated above their softening temperature. More specifically, the invention concerns the problem of positioning the glass sheets with respect to the tools such as the curving molds. The invention can be applied, for example, to the industrial manufacture of automobile glazings.
2. Description of the Related Art
The proper control of the quality of the output from a production line for curving glass sheets requires that it shall be possible to guarantee that each glass sheet is correctly positioned relative to the curving tool at the instant at which the latter takes the sheet over. This operation is fairly easy to achieve when it is performed cold, which is the case, for instance, for the majority of curving lines intended for producing laminated glazing panes, in which from the time of entry into the furnace the glass sheets are placed on their curving frame by settlement into position. In these cases, the referencing is typically achieved by sets of stops mounted on the curving tools. Much more strict precautions must, however, be taken when this taking-over is performed with a glass sheet that has already been reheated to its curving temperature and possibly toughening temperature, and therefore at a time at which its plasticity and consequently its sensitivity to shocks and friction are at the maximum.
When cold, on entering the reheating furnace, the glass sheet is charged onto the substantially horizontal conveyor in accordance with a very precise position. But its path along this conveyor, which is generally composed of a bed of rollers, is somewhat uncertain at least in consideration of the fact that in the most difficult cases, for example, an accuracy of positioning of the glass sheet of the order of one-tenth of a millimeter may be required. The discrepancies in trajectory may be analysed into slidings perpendicular to the axis of the conveyor and rotations about this axis.
It is possible to remedy perpendicular sliding by means of straight-edges or lateral stops moving in synchronism with the movement of the glass sheets; an example of such a system of lateral positioning is described in European Patent Application EP-A-367 670. These systems can operate very gradually and gently, and for this reason do not leave any imprint on the final plane. But these systems do result in wear of the roller facings. Moreover, in roller furnaces, it is generally found that the glass sheet maintains the lateral position acquired from the instant at which its temperature exceeds the softening point of the glass, which generally occurs at latest after it has completed two-thirds of its travel through the furnace.
In contrast, angular positioning systems are much less satisfactory, even though it has been possible to make considerable improvements to them in the past. In fact, the problem does not really reside in the systems themselves, but in the fact that the tendency to an axial deviation persists very late, virtually up to the taking-over by the shaping tool. This tendency can be explained, notably, by slipping on the rollers, which cannot be entirely eliminated, because it is absolutely imperative to prevent the rollers from marking the glass sheet in a permanent manner.
To minimize the consequences of these angular deviations, it is therefore imperative to perform the corresponding centering virtually at the same instant as taking-over, which is a nuisance from several aspects. The first reason is that the operation takes place at the instant at which, as indicated above, the sensitivity to marking of the glass is at its maximum; now by definition the centering of a glass sheet requires that it shall be acted upon and touched. Systems are indeed known which tend to eliminate the harmful consequences of this contact, for example systems which do not stop the glass sheet but only slow it down (EP-A-389 316) or which retract again as rapidly as possible (EP-A-389 317). But the expense of these systems frequently becomes higher the more efficient they are and in any case their sophistication increases the risks of incorrect functioning.
The second point is that all these systems are to some extent self-regulating or, more precisely, they act upon the glass sheet in a manner and/or for a period which depend upon the position of the sheet when it enters their field of action. The operator in charge of monitoring the production line has no means of estimating if this action has been performed deliberately. He will, most certainly, when this is not the case find out that the panes produced are not entirely in conformity with the requirements established but this non-conformity may possibly be explained by other reasons, such as incorrect setting of the pressing frame, for example, or more generally of the curving machine, with the result that there is a risk of not immediately identifying the source of the problems and of upsetting everything by a series of inappropriate actions.
Another harmful aspect of late centering is that, in a certain number of cases, the taking-over zone is already largely encumbered by the relevant tools and other devices; moreover, centering systems must be retracted, for example to enable the glass sheet to be pressed, and this again increases the number of possible sources of problems. Finally, since the positioning system stops or at least slows down the glass sheet, it leads to an increase in cycle times, which runs contrary to the objective of high output rates, particularly for relatively simple products.